1. Field of the Invention
The present invention relates to a shift lever bush which is mounted between a shaft portion formed in one of a pair of axially two-piece-divided shift lever members and a cylindrical portion formed in the other shift lever member.
2. Description of the Prior Art
There is a previously known shift lever bush of the type which is mounted between a shaft portion formed in one of a pair of axially (substantially vertically) two-piece-divided shift lever members (for example one located on the transmission side) and a cylindrical portion formed in the other shift lever member (for example another located on the shift knob side). This type of shift lever bush is intended for preventing the transfer of vibrations from the transmission side to the shift knob side.
For example, Japanese Utility Model Registration No. 2569721 Gazette discloses a shift lever bush. This shift lever bush includes: an inner cylindrical body which is mounted to a shaft portion of one of shift lever members by external interfit; an outer cylindrical body which is so disposed substantially coaxially with the cylinder axis of the inner cylindrical body as to surround an outer peripheral surface of the inner cylindrical body and which is mounted to a cylindrical portion of the other shift lever member by internal interfit; and a rubber elastic body, disposed between the inner cylindrical body and the outer cylindrical body, for connecting together these inner and outer cylindrical bodies. In the shift lever bush disclosed in the aforementioned gazette, the rubber elastic body is provided with an annular groove portion which opens to its cylinder axis direction end surface, which groove portion interfits with an annular stopper member. Further, a recessed portion is formed in a peripheral lateral surface of the groove portion on the outer cylindrical body side and, in addition, a projecting portion engageable with the recessed portion is formed in the stopper member. There is defined a clearance of a given amount between these projecting and recessed portions. Because of such arrangement, when the shift lever member on the shift knob side receives an operation load caused by a shift operation or select operation of the shift lever, relative displacement between the inner cylindrical body and the outer cylindrical body will not be regulated until the recessed portion of the groove portion and the projecting portion of the stopper member abut with each other. However, after the recessed portion and the projecting portion have engaged with each other, both relative displacement between the inner cylindrical body and the outer cylindrical body around the cylinder axis (displacement in the torsional direction) and relative displacement between the inner cylindrical body and the outer cylindrical body in a radial direction rectangular to the cylinder axis (displacement in the direction rectangular to the cylinder axis) are limited. Therefore, the spring characteristic of the shift lever bush disclosed in the aforementioned gazette is a two-stage characteristic for the torsional direction and for the direction rectangular to the cylinder axis with respect to its displacement amount.
However, in addition to the inner cylindrical body, the outer cylindrical body, and the rubber elastic body, the shift lever bush of the aforementioned gazette further requires the stopper member in order that its spring characteristic may have a two-stage characteristic, which results in the increase in component count. Besides, in this conventional shift lever bush, there is provided a stopper mechanism for placing and fixing the stopper member within the groove portion. This makes the construction complicated. As a result, the shift lever bush of the aforementioned gazette suffers inconveniences such as the increase in component count which results in the increase in manufacturing cost.
Further, the manufacture of the shift lever bush shown in the aforesaid gazette requires: firstly a step of performing integral vulcanization molding of the inner cylindrical body, the outer cylindrical body, and the rubber elastic body; a step of inserting, after locating the recessed portion of the groove portion and the projecting portion of the stopper member so as to circumferentially conform with each other, the stopper member into the groove portion; and a step of bending an opening edge of the inner cylindrical body and then bringing the opening edge portion into abutment with the stopper member to provide a stopper mechanism. Each step is extremely complicated and, besides, the number of steps is large, therefore producing such inconvenience that the manufacturing cost increases.
Bearing in mind the above-described circumstances, the present invention was made. Accordingly, an object of the present invention is to make it possible to manufacture a shift lever bush having a two-stage spring characteristic at lower cost.
In order to achieve the object, a first invention of the present application discloses a shift lever bush mounted between a shaft portion formed in one of a pair of axially two-piece-divided shift lever members and a cylindrical portion formed in the other of the pair of shift lever members. The shift lever bush of the first invention comprises: an inner cylindrical body mounted to the shaft portion of the one shift lever member by external interfit, an outer cylindrical body disposed substantially coaxially with a cylinder axis of the inner cylindrical body so as to surround an outer peripheral surface of the inner cylindrical body, and mounted to the cylindrical portion of the other shift lever member by internal interfit; and a rubber elastic body, fixedly attached to the outer peripheral surface of the inner cylindrical body, for connecting together the inner and outer cylindrical bodies. Further, an interfit fixing portion which is fixed to an inner peripheral surface of the outer cylindrical body by internal interfit and at least one projecting portion which becomes radially convex from the inner cylindrical body toward the outer cylindrical body are formed in the rubber elastic body, wherein the interfit fixing portion and the projecting portion are located at different positions in the cylinder axis direction, and a recessed portion, which becomes concave correspondingly to the shape of the projecting portion with a clearance of a given amount left between the recessed portion and the projecting portion, is formed at a position of the inner peripheral surface of the outer cylindrical body corresponding to the position of the projecting portion.
As a result of such arrangement, the inner cylindrical body mounted, by external interfit, to the shaft portion of one of the pair of shift lever members and the outer cylindrical body mounted, by internal interfit, to the cylindrical portion of the other shift lever member are interconnected at the interfit fixing portion formed in the rubber elastic body. Therefore, relative displacement between the inner cylindrical body and the outer cylindrical body in the cylinder axis direction is regulated by the interfit fixing portion. Accordingly, the spring characteristic of the shift lever bush in the cylinder axis direction becomes soft by making the length of the interfit fixing portion in the cylinder axis direction relatively short. As a result of making the spring characteristic of the shift lever bush in the cylinder axis direction soft, it becomes possible to impede the transfer of vibrations from the transmission side to the shift knob side, thereby achieving an excellent vibration proof effect.
On the other hand, when the shift lever member on the shift knob side receives an operation load caused by a shift operation or select operation of the shift lever, the inner cylindrical body and the outer cylindrical body will relatively rotate about the cylinder axis (the inner cylindrical body and the outer cylindrical body are twisted relative to each other) or will relatively be displaced in the radial direction rectangular to the cylinder axis.
At this time, with regard to the torsional direction, since there is defined a clearance of a given amount between the projecting portion of the rubber elastic body and the recessed portion of the outer cylindrical body, the inner cylindrical body and the outer cylindrical body relatively rotate about the cylinder axis by a corresponding angle to the clearance, with little resistance. This provides a soft initial spring characteristic. Thereafter, the projecting portion and the recessed portion engage with each other, thereby producing a hard spring characteristic. Accordingly, the shift lever bush exhibits a two-stage spring characteristic in the torsional direction.
On the other hand, also with regard to the radial direction, because of a clearance of a given amount defined between the projection portion and the recessed portion, at the positions where the projecting portion and the recessed portion are formed the inner cylindrical body and the outer cylindrical body are displaced relative to each other by a corresponding radial distance to the clearance with little resistance. This provides a soft initial spring characteristic. Thereafter, the projecting portion and the recessed portion abut with each other, thereby providing a hard spring characteristic. Further, with regard to a position circumferentially different from the formation positions of the projecting and recessed portions in the shift lever bush, there is defined a clearance of a given amount between the outer peripheral surface of the rubber elastic body and the inner peripheral surface of the outer cylindrical body. As a result of such arrangement, the inner cylindrical body and the outer cylindrical body are displaced relative to each other by a corresponding radial distance to the given-amount clearance with little resistance. This provides a soft initial spring characteristic. Thereafter, the outer peripheral surface of the rubber elastic body and the inner peripheral surface of the outer cylindrical body abut each other, thereby providing a hard spring characteristic. In this way, at any circumferential position, the spring characteristic of the shift lever bush for the radial direction is a two-stage characteristic.
As described above, the spring characteristic of the shift lever bush both for the torsional direction and for the radial direction becomes a two-stage characteristic, thereby providing improved operation feelings of the shift lever. Further, by virtue of the formation of the projecting portion of the rubber elastic body and the formation of the recessed portion of the outer cylindrical body, the spring characteristic of the shift lever bush according to the present invention has a two-stage characteristic. Because of this, the number of components required can be reduced and the construction becomes simplified, thereby reducing the cost of manufacture.
In a second invention according to the first invention, the recessed portion of the outer cylindrical body is formed by a pair of recesses which are circumferentially spaced from each other by a given distance in the outer peripheral surface of the outer cylindrical body and which are so recessed as to respectively project toward the inner peripheral surface of the outer cylindrical body and the inner cylindrical body is inserted to the inside of the outer cylindrical body, with the rubber elastic body fixedly mounted to the inner cylindrical body, and wherein during the insertion the interfit fixing portion of the rubber elastic body is interfittingly fixed, by press fit, to the inner peripheral surface of the outer cylindrical body. Further, the interfit fixing portion is formed at an end of the rubber elastic body on the side of a base end thereof in the direction of the insertion of the inner cylindrical body into the outer cylindrical body and an introduction portion, which becomes radially convex from the inner cylindrical body toward the outer cylindrical body and the amount of projection of which is smaller than that of the projecting portion, is formed at a position of the rubber elastic body located nearer to the side of a leading end of the inner cylindrical body in the insertion direction than the projecting portion.
In accordance with such arrangement, when manufacturing a shift lever bush of the present invention, the inner cylindrical body is placed in the inside of the outer cylindrical body, with the rubber elastic body fixedly mounted to the inner cylindrical body and, in addition, the interfit fixing portion of the rubber elastic body is press fitted into the inner peripheral surface of the outer cylindrical body. The inner cylindrical body and the outer cylindrical body are assembled together, only by these steps. This provides a simpler manufacturing process in comparison with prior art manufacturing processes. Also, the number of steps required is reduced, thereby reducing the cost of manufacture.
Further, the introduction portion is formed at a position of the rubber elastic body located nearer to the side of the leading end of the inner cylindrical body in the insertion direction than the projecting portion. Because of such arrangement, when assembling together the inner cylindrical body and the outer cylindrical body, the circumferential location of the projecting portion and that of the recessed portion automatically coincide, just by inserting the inner cylindrical body in the outer cylindrical body with the introduction portion in agreement in circumferential position with the recessed portion of the outer cylindrical body. This facilitates, when assembling together the inner cylindrical body and the outer cylindrical body, circumferential location of the inner cylindrical body and the outer cylindrical body. Therefore, the manufacture of a shift lever bush is further simplified, thereby reducing the cost of manufacture to a further extent. Further, the amount of projection of the introduction portion is set smaller than the amount of projection of the projecting portion. As a result of such setting, when placing the inner cylindrical body in the inside of the outer cylindrical body, the introduction portion is allowed to easily pass between a pair of recesses forming the recessed portion in the direction of the cylinder axis. This facilitates not only circumferential location of the inner cylindrical body and the outer cylindrical body but also assembly of the inner cylindrical body and the outer cylindrical body.
Further, the recessed portion of the outer cylindrical body comprises a pair of recesses which are so formed in the outer peripheral surface of the outer cylindrical body as to project toward the inner peripheral surface of the outer cylindrical body. This facilitates formation of the outer cylindrical body and reduces the cost of manufacture to a further extent.